BERLIN — Remember that harmless radioactive cloud that mysteriously drifted across Europe back in September? Turns out it may indeed have come from Russia after all — from an area that had radioactivity of about 1,000 times higher than normal, as officials there acknowledged for the first time Tuesday. Experts emphasized that the unusually high levels may still have been harmless.

What remains a mystery, however, is what produced this cloud. The most likely culprit, a serial offender nuclear reprocessing plant, still denies any connection.

It was Austria that first detected unusually high levels of radiation Oct. 3, with Germany confirming them the next day. Over the next two weeks, the levels went up and down and finally faded away over a vast swath of the continent.

France’s Institute for Radiological Protection and Nuclear Safety calmed fears this month, saying that the cloud of radioactive isotopes — Ruthenium-106 — had posed no health hazards. But the French researchers remained baffled by the cloud’s origins, and over the next few weeks, they calculated that it most likely came from deep inside Russia. Germany's governmental Agency for Radiation Protection came to the same conclusion.

At the time, Russian authorities denied the existence of a leak, and the state-owned Rosatom corporation said there had been no leaks in any of its plants and no unusual levels of radioactivity in the area.

On Tuesday, however, for the first time, Russia’s Meteorological Service confirmed that it recorded “extremely high contamination” with radioactive isotopes in the southern Urals region at the end of September, according to AP and AFP. Its researchers recorded radiation almost 1,000 times as high as normal levels there — higher than almost anywhere else in Europe.

The area is close to Kazakhstan, too, but attempts to narrow down the point of origin are now focusing on Russia.

Russia's confirmation Tuesday largely matched the earlier assessment by French authorities, but Rosatom continues to dispute that it is responsible for the high radiation levels, saying that “the recent Ru-106 emission which is being reported is not linked to any Rosatom site.”

“The published data is not sufficient to establish the location (country) of the pollution source,” Rosatom's statement said.

In their study, the French researchers said they believed that the Ruthenium-106 isotopes could have come from a medical research plant, a fallen satellite (of which there had been no reports) or a nuclear fuel reprocessing site.

The Mayak reprocessing plant in the Chelyabinsk region, which has already been fingered by analysts as a likely origin of the leak, was involved in several incidents in the past, including most recently a nuclear waste dump into a nearby river in 2004. In 1957, the plant triggered what environmental activists later called the world's “second biggest nuclear disaster in history” after a storage tank exploded and radioactive waste was spread across the region, contaminating tens of thousands of people.

Almost 30 years later, the extent of the catastrophic Chernobyl nuclear accident —  in Ukraine — was similarly covered up by Soviet officials who for days did not even acknowledge that the incident happened. The first researchers to raise alarm at the time were Swedish scientists. Defying international and domestic experts, the Soviet Union continued to dismiss the warnings as Western propaganda.

Amazing timelapse shows final installment of Chernobyl-arch over crippled reactor (EBRD/Novarka)

Tuesday’s announcement raises new questions about Russia’s commitment to transparency over nuclear issues and whether authorities have adequately dealt with this latest incident. It has prompted demands by Greenpeace’s Russian branch for a more “in-depth inquiry.” Radio Free Europe quoted local officials as saying that authorities did not inform them about a possible nuclear incident at the time.

Had a similar spike in radiation occurred in Europe, the French researchers wrote in the assessment this month, the government would likely have ordered a local evacuation.

Read more: